I haven't built my deck yet and it will likely be the last thing I do. My plan is to build one that is L shaped. This would be as wide as the bus in the rear area. From the rear wheels or so forward, it would be just cover about half (meaning just one side) so there is roof exposed for eventual solar or whatever I may decide on such a kayak rack. 5' wide is probably what I will go for in the narrow portion, which leaves 3' for the solar or kayak.
The rear area completely across gives a wide area for getting up the ladder and hauling/tilting stuff up to the roof rack.

I think the 2 general ways to mount a roof rack is to either bolt some vertical bars on the outside of the bus and put some 8ft long cross beams across the roof.
the other way is to lay down a metal/wood piece on either side of the roof, this piece run forward/backward along the edge of the roof and it bolted to the roof and then you put come cross pieces across the bus,
I choose to mount some vertical pieces of 2in square tube 11ga, they were 42in long each, bolted between each window, then I used the same tube at 8ft long to cross over the roof, at the top center of the bus the beams are 3in above the roof, I then put down 14ga plate steel for the floor, 3/8in grade 8 bolts glued for all the square tube connections.

I used a solar panel racking system using made by Unirac extruded aluminum rails down the length of each side. I have tied my deck boards and solar panels into these rails using standard SS solar mounting hardware. Super strong, super light, flexible, holds my deck and my panels, and its all aluminum and SS so it will never rust. I wish I had a pic. I will as soon as this damn rain lets up!

That's the system I used. The decking and panels also rest in the center, on the top of the apex of the roof. I did a support from the frame of the roof to the rail every 27" (which is the spacing on the bus frame)

Full length and full width always makes me think of people who are trying to haul a matress/box spring on the roof of a minivan. I picture the wind under it while going down the highway. Its one way to test adhesives. I've seen more than one surfboard come off a car roof and do flips down the road. I'm just saying. Think safety of the guy behind you even if your top speed might be 50-55.

Full length and full width always makes me think of people who are trying to haul a matress/box spring on the roof of a minivan. I picture the wind under it while going down the highway. Its one way to test adhesives. I've seen more than one surfboard come off a car roof and do flips down the road. I'm just saying. Think safety of the guy behind you even if your top speed might be 50-55.

Point taken, although I don't think that it would be that hard to securely fasten a deck to the bus. I am, however, concerned about the effect the wind might have on handling of the bus at highway speeds...